Although a thermopile type detector can transform electromagnetic radiant energy into an electric signal without using a chopping means over a rather wide range of wave lengths, it is necessary to keep the temperature of the cold junction of the thermopile constant or to correct the output of the thermopile depending on the measured temperature of the cold junction of the thermopile in order to obtain a stable output, because a minute difference in temperatures between the hot junction and cold junction of the thermopile resulting from an incidence of radiant energy onto said hot junction of the thermopile generates a thermo-electromotive force and thereby gives an electric signal in accordance with said difference in temperatures.
One common method by which the output of a thermopile has been corrected for the temperature of the cold junction of the thermopile is to carry out the correction on the basis of the room temperature (ambient temperature) in which the thermopile is present which temperature is measured by a thermistor having a positive temperature coefficient characteristic and installed in an electric circuit outside of the thermopile. However, a high accuracy of measurement can not be expected because the temperature of the cold junction does not always reach the room temperature quickly becuase it sometimes takes a long time until the temperature of the cold junction reaches a state of equilibrium with the room temperature.
It is thus necessary to measure the room temperature at a position as close as possible to the cold junction in order to secure a high accuracy of measurement.
Although it might be thought that the temperature of the cold junction of the thermopile could be measured by means of ready-made temperature response elements incorporated into the inside of the housing containing the thermopile, (usually housings such as To-5, To-18 type housings and the like), such response elements being thermistors, diodes, transistors and the like, strictly speaking, it is unavoidable that even in this case there will be somewhat of a gap between the temperature response element and the cold junction, and the temperature response element itself may have a large thermal capacity because of the presence of a case therefor, paint coated thereon, and the like, so that it takes a relatively long time until the temperature of the temperature response element reaches a state of equilibrium with the temperature of the cold junction, thus resulting in an inacccurate output of the thermocouple.